Sliding member

ABSTRACT

There is provided a sliding member that has sufficient wear resistance and good adhesion to a base formed of a sintered body. The sliding member includes a surface layer formed of a crosslinked fluoropolymer and a base that adheres closely to the surface layer. The base is a sintered body having a full density ratio in the range of 0.75 to 0.96 and is formed of a material having higher thermal conductivity than a fluoropolymer. The surface layer has a thickness in the range of 1 to 300 μm.

TECHNICAL FIELD

The present invention relates to a sliding member in which a sliding portion of a base formed of a sintered body contains a fluoropolymer. The sliding member has high wear resistance and can be suitably used in unlubricated bearings, oil pump rotors, cam rings, or the like.

BACKGROUND ART

Fluoropolymers are chemically very stable and have low adhesion and low frictional (low friction coefficient) properties. Because of these characteristics, fluoropolymers are widely used in various industrial products, such as seals and packing, and consumer products, such as cooking utensils.

Sliding members, such as unlubricated bearings, require low friction coefficients and often require heat resistance and chemical stability. Thus, it is anticipated that sliding members will be formed of fluoropolymers in the near future. However, sliding members require high wear resistance, whereas fluoropolymers have a wear problem. Thus, it is difficult to use fluoropolymers in sliding members unless the wear resistance of the fluoropolymers is improved.

It is known that the wear resistance of fluoropolymers can be improved by adding filler to the fluoropolymers. However, filler may impair the excellent inherent characteristics of fluoropolymers, such as low frictional properties. In this situation, Patent Literature 1 describes a method for improving the wear resistance of fluoropolymers by ionizing radiation and discloses a sliding member formed of a fluoropolymer subjected to ionizing radiation.

It has been thought that radiation impairs the mechanical characteristics of fluoropolymers. However, radiation under particular conditions can improve mechanical characteristics. For example, Patent Literature 2 discloses that ionizing radiation, such as electron beam radiation, at a dose in the range of approximately 1 kGy to 10 MGy in the absence of oxygen at a temperature of crystalline melting point or higher, preferably approximately 340° C., can suppress reduction in the elongation at break or breaking strength of polytetrafluoroethylene (PTFE) resulting from radiation, and, conversely, induce rubber elasticity with low crystallinity, and improve yield strength.

Sliding members have a problem in that an increase in the surface temperature of a sliding member due to heat generation resulting from sliding makes the sliding member more susceptible to wear. To address this problem, a method for manufacturing a sliding member by adhering a fluoropolymer closely to a heat dissipator made of a metallic material serving as a base is known (Patent Literature 3). In this sliding member, the heat dissipator can dissipate heat and prevent an increase in the temperature of the fluoropolymer due to heat generation resulting from sliding. In this case, however, in addition to high wear resistance of the fluoropolymer, good adhesion between the fluoropolymer film and the base is required.

CITATION LIST Patent Literature

-   PTL 1: Japanese Patent No. 3566805 -   PTL 2: Japanese Patent No. 3317452 -   PTL 3: Japanese Unexamined Patent Application Publication No.     2011-208802

SUMMARY OF INVENTION Technical Problem

In Patent Literature 3, the metallic material base is coated with the fluoropolymer. Considering more efficient heat dispersion, however, the base may be formed of a sintered body. Patent Literature 3 did not describe a base formed of a sintered body. Furthermore, unlike metallic materials, sintered bodies have a rough surface. Thus, the adhesion of a fluoropolymer film to sintered bodies will be different from the adhesion of a fluoropolymer film to metallic materials even if the fluoropolymer films are formed under the same conditions.

Accordingly, it is an object of the present invention to provide a sliding member that has sufficient wear resistance and good adhesion to a base formed of a sintered body.

Solution to Problem

As a result of extensive studies to solve the problems described above, the present inventors found that sufficient wear resistance and low wear as well as improved adhesion to a base formed of a sintered body can be achieved by adjusting the full density ratio of the sintered body of the base within a predetermined range.

The gist of the present invention consists in the following [1] to [4]:

[1] A sliding member that includes a surface layer formed of a crosslinked fluoropolymer; and a base that adheres closely to the surface layer, wherein the base is a sintered body having a full density ratio in the range of 0.75 to 0.96, the base is formed of a material having higher thermal conductivity than a fluoropolymer, and the surface layer has a thickness in the range of 1 to 300 μm.

[2] The sliding member according to [1], wherein the base is an iron-based sintered body.

[3] The sliding member according to [1] or [2], wherein the surface layer has a thickness in the range of 10 to 100 μm.

[4] The sliding member according to any one of [1] to [3], wherein the fluoropolymer is at least one selected from the group consisting of polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymers, and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers.

Advantageous Effects of Invention

The present invention can provide a sliding member that has sufficient wear resistance and improved adhesion to a base formed of a sintered body.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic perspective view of a thrust wear test (ring-on-disk wear evaluation) in examples and comparative examples.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described below. The present invention is not limited to these embodiments and examples. These embodiments and examples may be modified, provided that the gist of the present invention is not compromised.

A sliding member according to the present invention includes a surface layer formed of a crosslinked fluoropolymer and a base that adheres closely to the surface layer.

The fluoropolymer refers to a resin containing fluorine. The fluoropolymer that forms the surface layer is preferably polytetrafluoroethylene (PTFE), a tetrafluoroethylene-hexafluoropropylene copolymer (FEP), or a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) in terms of mechanical strength and chemical resistance. Among these, PTFE is more preferred because of its particularly high mechanical strength, chemical resistance, and heat resistance. The fluoropolymer may contain another component, provided that the gist of the present invention is not compromised. For example, PTFE may contain a minute amount of polymerization unit based on a copolymerizable monomer, such as perfluoro(alkyl vinyl ether), hexafluoropropylene, (perfluoroalkyl)ethylene, or chlorotrifluoroethylene. The fluoropolymer may be a mixture of two or more fluoropolymers.

The surface layer formed of the fluoropolymer must adhere to the base. When the surface layer peels off, and the base is exposed, the sliding member cannot perform its function.

Thus, in order to improve adhesion to the base, the fluoropolymer and the base are simultaneously exposed to ionizing radiation. No cross-linking or insufficient cross-linking results in low wear resistance and low mechanical strength of the fluoropolymer, and the sliding member cannot be used. The dose of ionizing radiation generally ranges from approximately 1 to 1500 kGy.

The surface layer formed of the fluoropolymer preferably has a thickness of 1 μm or more, more preferably 10 μm or more. A surface layer having an excessively small thickness is unfavorable because protrusions of the base appear on the surface layer. The upper limit of the thickness of the surface layer is preferably 300 μm, more preferably 100 μm. When the surface layer has an excessively large thickness, heat generated on the surface of the member by sliding is not easily conducted to the base, and it is difficult to prevent an increase in the surface temperature of the member. This tends to result in insufficient wear resistance. When the surface layer has a thickness in the preferred range, heat generated on the sliding surface by sliding can be effectively conducted to the base and is readily dissipated. This can suppress an increase in the surface temperature of the sliding member and improve wear resistance.

Wear resistance is often evaluated by a thrust wear test (ring-on-disk wear evaluation) by rotating a cylinder on a sample under pressure to measure wear. In particular, wear resistance and the coefficient of kinetic friction (μ) are often evaluated as a multiplier (critical PV) of a pressure (P) and a rotation speed (V) at which rapid wear occurs in this test method. A sliding member according to the present invention has a very high critical PV, good lubricity, high wear resistance, and a low coefficient of kinetic friction.

The base is formed of a sintered body that has higher thermal conductivity than the fluoropolymer. This sintered body is formed by heating an aggregate of a raw powder at a temperature lower than the melting point of the raw powder. More specifically, the sintered body is formed by charging a metal mold having a product shape with a raw powder, compressing the raw powder at a predetermined pressure, and sintering the resulting compact.

The raw powder may be a metal powder or a nonmetal powder. The metal powder may be an iron powder or a nonferrous metal powder. The iron powder may be a pure iron powder, an iron alloy powder, such as a carbon steel powder, or a partially sintered iron powder. The nonferrous metal powder may be a powder of a metal such as copper, nickel, manganese, chromium, or aluminum or an alloy not containing iron. The nonmetal powder may be a powder of nonmetal, such as a graphite powder or a ceramic powder.

The full density ratio of the base, that is, the ratio of the density of the sintered body that forms the base to the density of the metal that composes the base is preferably 0.75 or more, more preferably 0.80 or more. A full density ratio of less than 0.75 may result in insufficient strength of the sliding member. The upper limit of the full density ratio is preferably 0.96, more preferably 0.89. A full density ratio of more than 0.96 tends to result in low surface porosity and insufficient adhesion to the surface layer formed of the fluoropolymer.

The base has a larger volume than the surface layer formed of the fluoropolymer. The base has high heat resistance so as to withstand heat generated by sliding. When the sintered body that forms the base has lower thermal conductivity than the fluoropolymer of the surface layer, it is difficult to dissipate heat generated on the surface of the member by sliding and to prevent an increase in the surface temperature of the member. When the base has a small volume and accordingly small heat capacity, it is difficult to dissipate heat generated in the same manner and to prevent an increase in the surface temperature of the member.

The base preferably has a thermal conductivity of 0.001 cal/° C.·cm·s or more, more preferably 0.01 cal/° C.·cm·s or more, still more preferably 0.1 cal/° C.·cm·s or more.

The base is formed of a material having a higher thermal conductivity than a fluoropolymer. A fluoropolymer not containing filler has a thermal conductivity of approximately 0.0005 cal/° C.·cm·s (0.0005 cal/° C.·cm·s for PTFE). Thus, when the base has a thermal conductivity of less than 0.001 cal/° C.·cm·s, this may result in insufficient heat transfer from the surface layer formed of a fluoropolymer to the base. The base preferably has as high a thermal conductivity as possible.

As described above, the base has a larger volume than the surface layer formed of the fluoropolymer and preferably has as large a volume as possible in terms of heat dissipation. More specifically, when X denotes the thermal conductivity of the base expressed in cal/° C.·cm·s, and Y denotes (the volume of the base)/(the volume of the surface layer formed of the fluoropolymer), X·Y is preferably 0.005 or more, more preferably 0.05 or more, still more preferably 0.5 or more.

Examples of the material for the base and the thermal conductivity of the material are as follows: iron: 0.18 cal/° C.·cm·s, aluminum: 0.53 cal/° C.·cm·s, and ceramic (brick): 0.07 cal/° C.·cm·s.

A process for manufacturing a sliding member according to the present invention will be described below.

First, a base having a predetermined shape is formed. The shape may be tabular, convex, or concave, or may be cylindrical or tubular and have a sliding portion on the outer surface thereof, or may be tubular and have a sliding portion on the inner surface thereof.

A fluoropolymer is then applied to a surface of the base, that is, a portion that will become a sliding portion to form a fluoropolymer layer. The fluoropolymer may be applied by a method of applying a fluoropolymer film; a powder coating method, for example, an electrostatic coating method using a fluoropolymer powder or a method of spraying a fluoropolymer powder; or a method of applying a fluoropolymer dispersion (a liquid containing a fluoropolymer powder uniformly dispersed in a dispersion medium) and removing the dispersion medium by drying.

Among these, a method of applying a fluoropolymer dispersion is preferred because this method can easily form a fluoropolymer layer having a uniform thickness. In the case of fluoropolymers that are soluble in a solvent, a fluoropolymer solution may be applied, and the solvent may be removed by drying. However, this method cannot be applied to fluoropolymers that are insoluble in a solvent, such as PTFE.

In a method of applying a fluoropolymer dispersion, the dispersion medium may be a mixed solvent of water and an emulsifier, water and an alcohol, water and acetone, or water, an alcohol, and acetone. After a fluoropolymer dispersion is applied, the dispersion medium is removed by air drying or hot-air drying. Removal of the dispersion medium by drying results in a film formed of the fluoropolymer powder.

After the fluoropolymer coating film is formed by the application or the like, the fluoropolymer coating film is fired at a temperature equal to or higher than the melting point of the fluoropolymer, and the fluoropolymer powder is fused and forms a fluoropolymer layer. The firing is preferably performed at a temperature in the range of 350° C. to 400° C. The dispersion medium may be removed in the firing step without the drying step.

A surface of the fluoropolymer layer thus formed is then irradiated with ionizing radiation to cross-link the fluoropolymer. When the combination of the fluoropolymer and the base material is appropriate, the cross-linking also improves the adhesion between the fluoropolymer layer and the base.

For cross-linking, a surface of the fluoropolymer film is irradiated with ionizing radiation in an oxygen-free atmosphere, more specifically, in an atmosphere having an oxygen concentration of 1000 ppm or less, preferably 10 ppm or less, at a temperature in the range of the crystalline melting point of the fluoropolymer to approximately 400° C., preferably at a temperature 0° C. to 30° C. higher than the crystalline melting point of the fluoropolymer. The radiation dose generally ranges from 1 to 1500 kGy, preferably 100 to 1000 kGy.

The firing and ionizing radiation may be simultaneously performed. An excessively low temperature atmosphere inhibits the cross-linking reaction of the fluoropolymer. An excessively high temperature atmosphere, particularly having a temperature of more than 400° C., promotes thermal decomposition of the fluoropolymer and impairs material properties. A radiation dose of less than 1 kGy results in an insufficient cross-linking reaction and no characteristic improvement. A radiation dose of more than 1500 kGy tends to result in an increased decomposition rate of the fluoropolymer.

Examples of ionizing radiation for use in the cross-linking of fluoropolymers include charged particle beams, such as electron beams and high-energy ion beams, high-energy electromagnetic waves, such as gamma rays and X-rays, and neutron beams. Electron beams are preferred because electron beam generators are relatively inexpensive and can produce high-power electron beams, and the degree of cross-linking can be easily controlled with electron beams.

The adhesion of a surface layer of a sliding member manufactured by the method described above can be measured by a cross-cut test. The cross-cut test is a test method described in JIS-K-5400 (1998). More specifically, 100 squares are scratched on a surface layer, and a tape is repeatedly attached to and peeled from the surface layer. The number of squares that remain on the surface layer is counted. 99/100 or more means that 99 or more squares of the 100 squares remain on the surface layer.

Poor adhesion between the surface layer and the base tends to result in insufficient contact (adhesion) between the surface layer and the base. In particular, sliding is likely to cause problems, such as the formation of voids. In particular, the formation of voids due to insufficient contact makes it difficult to conduct heat generated on the surface layer to the base and to prevent an increase in the surface temperature of the member. This tends to result in insufficient wear resistance. Thus, the surface layer preferably does not peel off from the base during 100 or more repetitions in the cross-cut test.

A sliding member according to the present invention has a low friction coefficient similar to the friction coefficient of known sliding members formed of a fluoropolymer and higher wear resistance than the known sliding members. Thus, a sliding member according to the present invention can be suitably used in applications that require high wear resistance, such as unlubricated bearings for use in industrial machinery and consumer products.

EXAMPLES

The present invention will be further described below with examples. First, the evaluation method will be described below.

<Evaluation Method> [Measurement of Wear Resistance]

The wear resistance of a fluoropolymer coating was evaluated by a thrust wear test (ring-on-disk wear evaluation, Suzuki wear evaluation). More specifically, as illustrated in FIG. 1, the wear of a test sample is measured by rotating a metal cylinder (mating shaft) on the test sample under a predetermined load (contact pressure: P) and at a predetermined speed (rotation speed: V).

The mating shaft was an S45C cylinder having an outer diameter/inner diameter=11.5/7.4. Wear was measured under dry (greaseless) lubrication conditions. The rotation speed (V) was constant at 1800 rpm. The critical PV (P·V at which rapid wear occurs) was determined by changing the contact pressure (P). The critical PV is listed in Table, wherein a circle denotes 100 MPa·m/min or more (good), a triangle denotes 1 to 100 MPa·m/min (fair), and a cross denotes less than 1 MPa·m/min (poor). With respect to lubricity, the coefficient of kinetic friction (μ) is listed in Table, wherein a circle denotes less than 0.5 (good), and a cross denotes 0.5 or more (poor).

[Measurement of Adhesion]

The peel resistance of a fluoropolymer coating was measured by a cross-cut test. More specifically, 100 squares were scratched on a fluoropolymer coating sample, and a tape was repeatedly attached to and peeled from the sample. The number of squares that remained on the sample was counted. The results after 10 repetitions are listed in Table, wherein a cross means that all the 100 squares peeled off from the sample (poor), a triangle means that 1 to 99 of 100 squares peeled off from the sample (fair), and a circle means that all the 100 squares remained on the sample (good).

[Measurement of Strength]

A tensile test was performed according to JIS Z 2241. The results are listed in Table, wherein a circle denotes a tensile strength of 300 MPa or more (good), and a dash (-) means unmeasurable. Members having a tensile strength of 300 MPa or more can be used as structural members of automobiles or the like.

Examples 1 to 3, Comparative Examples 1 to 6

A fluoropolymer dispersion (manufactured by Daikin Industries, Ltd.: D10-FE, resin type: PTFE) was applied to an iron sintered material (2.0% Cu-0.8% C-Fe) having a density listed in Table and a thickness of 20 mm, was dried, and was fired in a nitrogen atmosphere at 380° C. for 10 minutes. Thus, the iron sintered material was coated with a fluoropolymer film having a thickness of 15 μm. The iron sintered material coated with the fluoropolymer film was then heated to 330° C. in a nitrogen atmosphere (oxygen concentration: 5 ppm) and was irradiated at 300 kGy using an irradiation apparatus manufactured by Nissin Electric Co., Ltd. (Sagatron: accelerating voltage 1.13 MeV). A test sample was prepared in this manner.

The test sample was subjected to the tests described above. Table shows the results.

Steel used in Comparative Example 2 was SNCM630 steel. In the same manner as described above, a fluoropolymer film was formed, and electron beam irradiation was performed.

TABLE Comparative example Example Comparative example 1 1 2 3 2 3 4 5 6 Base Material Iron based Iron- Iron- Iron- Steel Iron- Iron- Iron- Iron- sintered based based based sheet based based based based body sintered sintered sintered sintered sintered sintered sintered body body body body body body body Density 5.6 6.4 6.8 7.2 7.8 6.4 6.8 7.2 6.8 (g/cm²) Full density 0.72 0.82 0.87 0.9 1 0.82 0.87 0.9 0.87 ratio Coating Yes Yes Yes Yes Yes Yes Yes Yes No Presence Electron Yes Yes Yes Yes Yes No No No No beam radiation Physical Adhesion to ◯ ◯ ◯ ◯ Δ Δ Δ Δ — properties base (cross- cut test) Wear ◯ ◯ ◯ ◯ ◯ X X X X resistance (Critical PV) Lubricity ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ X (kinetic friction coefficient μ) Tensile — ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ strength

These embodiments of the present invention are only examples. The scope of the present invention should not be limited to these embodiments. The scope of the present invention is defined by the appended claims and embraces all changes that fall within the scope of the claims and the equivalents thereof. 

1. A sliding member comprising: a surface layer formed of a crosslinked fluoropolymer; and a base that adheres closely to the surface layer, wherein the base is a sintered body having a full density ratio in the range of 0.75 to 0.96, the base is formed of a material having higher thermal conductivity than a fluoropolymer, and the surface layer has a thickness in the range of 1 to 300 μm.
 2. The sliding member according to claim 1, wherein the base is an iron-based sintered body.
 3. The sliding member according to claim 1, wherein the surface layer has a thickness in the range of 10 to 100 μm.
 4. The sliding member according to claim 1, wherein the fluoropolymer is at least one selected from the group consisting of polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymers, and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers. 